In many types of radiation exposure processes a film is used to record an image. It is often necessary to provide some indication of the location or size of the image that appears on the film. It has been a common practice to use a conventional ruler to measure distances directly on the film after exposure and development. Also, it is common practice to place a ruler, a measuring tape, or other measuring reference object with graduation marks that are opaque to x-rays against or on the body and then take an x-ray. The graduation marks show up as an image on the developed x-ray film.
X-rays are important in giving detailed information about the image that appears on the film after developing. However, it is important that the amount of radiation that a patient is subjected to and that the radiation is directed toward the correct part of the patient's body during an x-ray, a scan, or radiation treatment for cancer.
High dose-rate brachytherapy involves the temporary placement of a small, almost point-sized, radioisotope radiation source within a living body. This is done for the purpose of irradiating and killing cancer cells. The characteristics of the radiation source are chosen so that cells close to the source receive very high radiation doses, whereas the dose to tissue a few millimeters away is much lower, below the threshold for permanent damage. The radiation source is usually affixed to the end of a thin wire that runs within a catheter. For treatment, the catheter is inserted into the body, passing through the treatment site. Since placement of the source close to the cancer cells is of extreme importance, it is necessary to validate that the source will move accurately to a pre-determined position within the catheter. Prior to treatment, it is necessary to validate that the source can be positioned with the requisite accuracy. Typically, this is done by fixing a strip of radiation-sensitive film within a test fixture. The radiation source is passed along the test fixture and stopped at predetermined points. The source dwells at predetermined points for a significant time to locally expose the radiation-sensitive film. At the end of the test the film is retrieved and measurements are made with a scale to demonstrate that the source has stopped at the correct predetermined positions.
X-ray computed radiography is an important and widely-used modality in medical and security imaging. For purposes of quality assurance, it is important to routinely measure the slice thickness, i.e., the width of the x-ray beam used in the examination. In order not to expose a patient to unnecessarily high levels of radiation during a CT exam, it is especially important to establish that the slice width is within CT machine-operating tolerances. The conventional way of establishing slice width is to place a piece of silver halide film in the CT machine and expose the film to a number of slice widths. Thereafter, the film is developed and a ruler is used to measure the width of the exposed areas.
The disadvantage of silver halide film is that the user cannot directly observe the position of the latent image of a slice-width exposure before repositioning the film to make second, third and fourth exposures, etc. As a result, there is a substantial likelihood that one or more exposures will overlap and the test exposures have to be repeated. Alternatively, the user could expose a single slice on a piece of film. However, this is wasteful since the slice thickness is frequently between 5 mm and 20 mm wide while the film is 8″×10″, or greater, in size.
A common way to calibrate the dose response of radiation sensitive film is to place the film between two solid blocks and position the film so that it is parallel to a beam of radiation. The radiation dose to the film decreases with distance from the surface of the blocks upon which the radiation is incident. This is due to attenuation of the radiation in proportion to depth. The doses at particular distances from the incident surface are usually determined by using a primary measurement device such as an ion chamber placed at known depths in the solid blocks. When the film is exposed, the position of the film relative to the incident surface is carefully noted. After exposure, optical absorbance measurements are made on the film. Knowing the positions of the optical density measurements relative to their depths below the incident surface it is possible to relate the optical density of the film to the radiation dose at that depth and so construct a depth-dose curve.
In determining the depth-dose curve, optical absorption measurements may be made with a densitometer. However, this is a very laborious procedure requiring dozens of individual measurements in order to construct a detailed and accurate depth-dose response curve. A more convenient way to obtain the optical absorption data is to measure the film with an optical scanner. However, while this simplifies the collection of the absorption data, it introduces a greater uncertainty in knowing the depth corresponding to that measurement point because the scanner introduces spatial distortions into the scanned image.
In obtaining a radiograph of a patient, it is important that the portion of the patient that is to be examined be positioned in the center of the x-ray field. As a means of doing this, a light field is established in the radiography or mammography unit coincident with the x-ray field. The light field is projected onto the patient and the patient is then positioned so that the sight of the radiographic examination is centered within the light field.
In using the light field as a means for positioning patients, it is important to establish from time-to-time that the light field and x-ray fields are coincident. This test is commonly performed by a medical physicist one or more times per year on each radiography unit. The conventional way to do such a test was to align one or more pieces of silver halide film in light-proof envelopes so that they span the edges of the light field on all four sides. The edge of the light field is marked by pricking the envelopes with a small pin, thereby exposing the film to light. The film is then exposed to the x-ray beam. Following this, the film is taken to a darkroom and processed. Two problems can frequently occur. Thus, the pin-pricks may be too large and the film becomes overexposed making it difficult to accurately locate the edge of the light field. Another problem can occur if the film moves within the light-proof envelope after the pin-pricks have been made. A further inconvenience is that the film must be taken to a darkroom for development. Yet another inconvenience is that a ruler must be used to measure the positions of the pin-pricks and the edges of the x-ray field and determine the alignment of the light field with the radiation field.
Accordingly, there is a need for a radiation sensitive material comprising a support and a radiation sensitive composition that further includes a measuring scale to address the foregoing issues with the prior art. More specifically, there is a need for a radiochromic, self-developing film media that includes a measuring scale.